Conversion from Decapascals to Newtons per square meter

Information about the Decapascal (daPa)

The decapascal is a unit of pressure equal to ten pascals. Its symbol is daPa. Deca is the least used prefix in the metric system almost everywhere, but the decapascal is an exception, because one clinical discipline adopted it and has kept it: audiology measures the pressure inside the middle ear in decapascals.

Tympanometry is the test in question. A probe seals the ear canal and varies the air pressure in it while measuring how much sound the eardrum reflects, and the resulting curve is plotted against pressure in decapascals. The sweep runs from about plus 200 to minus 400, and the position of the peak shows the pressure at which the eardrum moves most freely, which is normally the pressure of the middle ear itself.

That single graph carries a great deal of clinical information. A peak near zero means the middle ear is at the same pressure as the room, which is the healthy state. A peak displaced towards minus 200 or beyond suggests the Eustachian tube is not equalising properly, and a flat curve with no peak at all suggests fluid behind the eardrum. Each of those readings is a position on a decapascal scale.

The unit was chosen for the same reason the hectopascal was chosen in meteorology. Middle-ear pressure had long been recorded in millimetres of water, and one millimetre of water is 0.98 decapascals, so switching to the metric unit left every clinical number and every published reference range almost unchanged. A change of unit that shifts the figures by two per cent is one that practitioners will actually accept.

Outside audiology the decapascal is essentially unused. Ventilation and building airtightness work at similar pressures, an airtightness test being run at fifty pascals, but those figures are written in pascals, and everything above them moves to hectopascals or kilopascals. The gap between one and a hundred pascals is otherwise unclaimed.

For scale, ten pascals is the pressure a light breeze exerts on a wall, or the difference between the two ends of a well-sealed corridor when a door opens. It is also roughly the pressure change from riding a lift up one floor, which is why ears sometimes need clearing in a tall building.

One decapascal equals 10 pascals, 0.1 hectopascals, about 1.02 millimetres of water, or about 0.00145 pounds per square inch.

Information about the Newton per square meter (N/m²)

The newton per square metre is a unit of pressure equal to one pascal. Its symbol is N/m². The two are not merely equivalent but identical: the pascal is the name given to this combination, and before the General Conference on Weights and Measures adopted that name in 1971 the SI unit of pressure had no name at all and was written out in full.

Both forms survive because they do different work on the page. The pascal is compact and reads as a unit in its own right, which suits a measurement. The newton per square metre shows its dimensions, which suits a calculation, because it makes visible that multiplying by an area in square metres will give a force in newtons.

Structural engineering leans on the second property constantly. Floor loads are specified in kilonewtons per square metre, with about 1.5 for a dwelling, 3 for an office and 5 for a place of assembly, and multiplying that figure by the floor area gives directly the load in kilonewtons that the beams must carry. Written as kilopascals the same numbers would be correct but would hide the step.

Snow and wind follow the same convention. Snow load is given in kilonewtons per square metre, from a few tenths in a mild climate to several in the mountains, and wind pressure on a facade likewise. Because those loads are combined with dead weight, which is naturally a force, keeping everything in newtons avoids the need to convert anything.

The construction repeats one prefix down. A newton per square millimetre is exactly one megapascal, which is why material strengths appear on drawings as N/mm² as often as MPa. The pattern is worth recognising: whenever a document writes force over area rather than naming a pressure unit, it is because the writer expects the reader to multiply.

Nothing else distinguishes the two forms. Any value in newtons per square metre can be written as pascals without change, and any conversion table treats them as one entry. The choice is a matter of what the number is about to be used for.

One newton per square metre equals 1 pascal, 0.01 millibars, 0.00001 bar, or about 0.000145 pounds per square inch.